In industrialized nations, colorectal cancer is the second leading cause of deaths from malignancy. In the United States, almost 150,000 people are found to have colon cancer annually and it causes approximately 60,000 deaths annually. Only lung cancer causes more deaths. Colon cancers are preventable because they usually begin as benign polyps which grow slowly for five to ten years before becoming cancerous. If these polyps are detected and removed, the risk of developing colon cancer is greatly reduced.
Unfortunately, widespread colorectal screening and preventive efforts are hampered by several practical impediments, including limited resources, methodologic inadequacies, and poor patient acceptance leading to poor compliance. In addition, a fecal occult blood test (FOBT) fails to detect the majority of cancers and pre cancerous polyps. Since sigmoidoscopy only examines a portion of the colon, it also misses many polyps. The accuracy of barium enema varies among centers and is therefore not always reliable.
Therefore, there is a need for a new test which can be used to screen for pre cancerous colon polyps. Like all screening tests, this new test must be relatively inexpensive, minimally invasive, and highly specific.
A technique using helical computed tomography (CT) to create computer simulated intraluminal fights through the colon was proposed as a novel approach for detecting colorectal neoplasms by Vining D J, Shifrin R Y, Grishaw E K, Liu K, Gelfand D W, Virtual colonscopy (Abst), Radiology Scientific Prgm 1994; 193(P):446. This technique was first described by Vining et al. in an earlier abstract by Vining D J, Gelfand D W, Noninvasive colonoscopy using helical CT scanning, 3D reconstruction, and virtual reality (Abst), SGR Scientific Program, 1994. This technique, referred to as “virtual colonoscopy”, requires a cleansed colon insufflated with air, a helical CT scan of approximately 30 seconds, and specialized three-dimensional (3D) imaging software to extract and display the mucosal surface. The resulting endoluminal images generated by the CT scan are displayed to a medical practitioner for diagnostic purposes.
The technique of reformatting 2D cross sections perpendicular to the colon midline is also described in U.S. Pat. No. 5,458,111, issued Oct. 17, 1995 to Coin. However, direct interpretation of the cross-sectional images is difficult because a scan of the colon consists of several hundred axial tomograms. Without advanced image manipulation tools, interpretation of the colon's complex three dimensional shape from these cross sections alone is very difficult for a medical practitioner.
One approach to improve accuracy involves production of reformatted 2D images at cross sections and orthogonal angles to the colon midline. These reformatted 2D images can provide complimentary diagnostic information when viewed with corresponding intraluminal 3D images. Exam efficiency can be improved with innovative 3D rendering techniques that allow fast, interactive evaluation on low priced computer hardware.
Retained fluid in the lumen of a colon is also a commonly encountered problem that can obscure lesions. Prone imaging in addition to standard supine imaging is often required to visualize obscured colonic segments. Although the colorectum is often seen optimally with combined supine and prone views, twice as much interpretation is required with both acquisitions.
Furthermore, the current CTC utilizes an interactive combination of axial, reformatted 2D and 3D images (from an endoluminal perspective) that are generated in real time. Navigating the colorectum within this “Virtual Endoscopy” (VE) metaphor is tedious. To alleviate this problem and free the radiologist to concentrate on diagnosis, a two-pass approach has been adopted. In the first pass, the colon is semiautomatically navigated and its midline is recorded. Then in a second pass, the radiologist moves the view point (virtual endoscope tip) along this midline. This second pass takes from several minutes per scan.
To reduce interpretation time of Computed Tomographic Colography (CTC), additional volume rendered displays have been developed which show longer segments of the colon in formats analogous to views which may be seen at autopsy. These include a technique called “Planar Virtual Pathology” (PVP) which uses image planes of the two longitudinal transcolonic sections as cut planes within isometric volume rendered images. Scenes are rendered with rays passing orthogonally through these planes from both sides. Each of the four resulting images depicts approximately one-half of the inner surface of a 12 cm bowel segment. Interpretation is performed by viewing these colon segments at approximately 8 cm intervals. In this way, the entire colon can be rapidly examined with a minimum of navigational input from a user such as a radiologist.
Other virtual pathology views have also been developed. These include Cylindrical Virtual Pathology (CVP) and Mercator Virtual Pathology (MVP). CVP views are formed by casting rays perpendicular to a straight line which approximates a segment of the colon midline. These views are analogous to the result of splitting a segment of excised colon and opening it to expose its inner surface. Interpretation can be performed interactively or by viewing a series of pre-computed CVP views as described for PVP above. With MVP, views are generated by casting rays from a single viewpoint in all directions. The resulting view is a panoramic image. Interpretation of CVP is performed interactively in a manner similar to that of VE.
In resolving the above problem, volume rendered endoluminal perspective views of the colon have been explored. However, available workstations were not fast enough to do the required rendering interactively. One of the methods of addressing this challenge is to render six images at each point along the colon midline. These images are aligned with the image coordinates and each has a ninety degree field of view. Virtual Endoscopy (VE) views with arbitrary camera orientation and view field can be derived from these images in real time using simple image warp and merge operations. These panoramic scene sequences make it possible to visualize the entire endoluminal surface without further manual navigation.
Additional panoramic projections have been developed. They are volume rendered scenes. However, they are quite different from other perspective volume renderings. In conventional perspective rendering techniques, pixel coordinates map the angle (or the tangent thereof) between the cast ray and the central axis of scene. The range of angles (field of view) is usually limited to around +/−45 degrees. The present invention produces images where these angles range from +/−90 degrees in the y-axis (latitude) and 0 to 360 degrees in x-axis (longitude). The result is a panoramic endoluminal view similar to a mercator projection map. This is called Mercator Virtual Pathology (MVP). Another technique provided by the present invention is to map angles of view from 0 to 360 degrees along the x-axis but to move the point of view for each row in the image. This forms a cylindrical projection. This is called Cylindrical Virtual Pathology (CVP). The advantage of each of these techniques of the present invention is that it shows larger regions of the colon in each rendered scene than previous virtual colonographic images. They also simplify navigation by eliminating two or three of the degrees of freedom.
Therefore, there is a need for techniques which provide efficient and accurate evaluation of the colon using helical CT data. There is a need for displaying coronal, sagittal, and axial views of the colon. There is also a need for displaying a three dimensional image of the colon. There is a further need for displaying an unfolded or open view of the colon.
Yet there is a need to further reduce interpretation time of the Computed Tomographic Colography. There is also a need for an improved display mode which enables a user to interpret long segments of the colon at one time and to reduce interpretation time for Computed Tomographic Colography.